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Soil Injection: The Smart, Non-Invasive and Environmentally Friendly way to Boost Drainage and/or Save Trees in your Garden
Soil injection is a method of using specialist tools, powered by compressed air, to create fissures in the subsoil which are then filled with dry or wet material to achieve a specific outcome.
We specialise in injecting dry material—namely Terramol, enriched biochar, or a mixture thereof—to improve drainage in gardens and boost or recover the health of ailing plants and trees.
Soil injection is used extensively by world-renowned centres of excellence, including the Royal Botanic Gardens, Kew, which pioneered the concept of aeration to combat declining tree health. It is also common in sporting venues that require non-invasive ways to maximise the quality of playing surfaces (e.g. professional stadiums, training facilities, golf clubs, etc).
As a drainage solution, soil injection is both cost-effective and environmentally friendly. Unlike alternatives such as French drains, which rely on discharging collected water (typically into the sewer network, compounding flooding issues), soil injection aims to boost the absorbance and water retention of existing green spaces. This reduces the potential for local flash flooding without placing additional strain on the broader network.
Soil injection—especially when leveraged across London’s 200+ acres of green squares (many of which sit above otherwise poorly draining, clay-rich subsoils)—can contribute significantly to Sustainable Drainage Systems (SuDS) and “Sponge Cities”, a concept deployed successfully in China and the tropics to deal with summer rainfall.
The Soil Injection Process
1: The needle hammers itself between 30-60cm deep into the soil (the process can be repeated at multiple heights if desired)
2: Compressed air is blasted several times out of the needle at over a 100psi, with an airflow of up to 2,500 cubic feet a minute* without damaging any nearby tree roots
3: The Air Blasting creates a series of radial fissures in the subsoil to a depth of up to 1.2m.
4. Fine dry material (up to 3mm in diameter) is then blasted out of the same needle, with between 0.75-2kg of material injected into each injection site*
*Airflow and Quantity of Material Injected dependent on strength of fuel-drive air compressor and type of material used. Access restricted sites may necessitate a smaller compressor with lower airflow and material transfer.
The injection procedure is then repeated once for every 1 square meter, with injection holes separated (x and y) by 1 meter.
For drainage applications, we will encourage Gardens to inject into lawns, the beds around significant trees, and into permeable paths where it won’t compromise any weed-membrane (e.g. old Gravel paths).
We will typically price per injection, with discounts based on the number of injections required. As a broad estimate:
- Root Protection Area of a Mature Plane Tree = c.150 sqm / 150 injections and 112.5-300kg of material injected
- A half-acre Garden = c.2,023 sqm / 2,023 injections and 1.5-4 tonnes of injected drainage/nutrient material
- A 2 acre Garden = c.8,094 sqm / 8,094 injections and 6-16 tonnes of injected drainage/nutrient material
A key benefit of the Soil Injection process is that it can be incremental. Gardens looking to increase their drainage to protect against issues like Flash Flooding can target specific areas prone to water pooling (lowest points) and run off (sloping lawns), before instructing further works after assessing the impact.
Soil Injection for Drainage
The Basics of Compaction and Poor Drainage
Many London Squares suffer from poor drainage owing to common factors including but not limited to:
- A high clay content within a meter of the surface preventing absorption of water in the Summer, and leading to bogs in the Winter – Clay soils are slow to absorb water, though also slow to release/redistribute water one damp. After a dry spell, clay soil becomes hard and compacted. Its fine particles shrink and create tight bonds, forming a crust that repels water, causing runoff instead of absorption. Re-wetting becomes slow and drastically uneven, with the boggiest sections of the Garden, typically low points where the run-off collects, retaining significantly more water for far longer than they would in low-clay-content soils (where the moisture would be better distributed).
- Pollarding/Removal of trees (typically to reduce subsidence risks) reducing water removal in the Winter – in high-clay-content areas it is typical for trees to be heavily pollarded (or even removed) specifically to reduce (or forgo) their transportation process (water is pulled from the soil and released through the leaves), thus reducing the risk of subsidence to nearby properties. This also, however, removes the ability of trees to reduce water content in sodden areas.
- Compaction of Lawns through Footfall, Events, Ride on Machinery, and even Heavy Goods Vehicles (e.g. Scaffolding Trucks) – all of which compress the soil, reducing its ability to absorb water. Damp soils are particularly prone to compaction, making high-clay gardens highly susceptible throughout the Winter months.
How Injecting Terrmol Can Help
The Primary Objective of soil injection is to create and preserve channels that allow for the greater transportation of water and oxygen in all directions throughout layers of suboptimal soil, thus increasing consistent absorption throughout the subsurface. This not only helps the ground absorb surface water, though also ensure the water is then distributed throughout the soil, rather than concentrated in a specific spot.
Terramol is a trade name for a horticultural product made from calcined moler clay, a type of diatomaceous earth that has been heat-treated to harden its structure. It’s commonly used in soil mixes, particularly for improving aeration, drainage, and water retention balance. Specifically, Terramol improves Soil Drainage by:
Improving Structure: It breaks up compacted soils, creating air pockets and allowing better root growth.
Enhancing Drainage Channels: Water flows more freely around its particles, reducing waterlogging.
Regulating Soil Moisture: Absorbs excess water and releases it slowly back to the plant roots, helping buffer against over- or under-watering.
Resisting Soil Compaction: Its rigid, granular form keeps the soil loose and friable over time
By better regulating and distributing the water content in even the most clay-ridden soils, Terramol can reduce both the severity of bogging in low-points of the Garden, whilst also reducing the frequency and stubbornness of water-repellent soil structures in the rest of the Garden.
The ambition is to get the soil structure as close as we can to a natural sponge, that absorbs and evenly distributes water during a deluge, then allows for it to be gradually drawn out over time.
Maintaining Porous Lawns to Boost Absorption
After a Soil Injection process, any Lawn will enjoy a period of above-average absorption, this is due to the presence of the large injection holes (backfilled with material such as Terramol) running to the surface.
Over time however, Lawns should be expected to grow over the injection holes, reducing the above average absorption experienced just after injection. The injected material will however not breakdown, it will stay in the soil preserving drainage channels.
The question is therefore what simple maintenance can be undertaken to improve the access to those channels from the surface, long after the injection works are complete?
Unattended, Lawns will naturally develop a thick thatch layer which keeps water on the surface and reduces absorption. It is therefore recommended that Gardens instructing Soil Injection to improve drainage, follow up any injection works with a regular (at least once every two years) process of scarifying the Lawn.
Scarifying Lawns every Easter will dramatically reduce the levels of built-up moss and thatch, whilst boosting surface absorption, and activating the drainage channels of the lawn, keeping otherwise water-resistant clay-rich soils moist for longer.
All areas of Lawn should be ideally gone-over twice by a scarifier, with the second pass at least 45-degrees offset from the first. As part of our stay-in-our-lane mantra, we would rather such works are undertaken by each Garden’s retained Gardener / Contract Gardening firm, however we are available to undertake such maintenance works when required.
Supporting Sustainable Drainage Systems (SuDS) and Sponge-City Concepts
Within past decades, as climate change and urban flooding have become increasingly urgent challenges, drainage systems designed specifically for environmental sustainability have become more popular in both academia and practice.
Modern London, with its many impermeable surfaces, is overly reliant on its Victorian Sewer Network, that can become quickly overwhelmed by rain run-off during flash flooding events. During low tides, the Sewer Network responds by discharging into the Thames (at huge environmental cost). During high tides however, the Sewer Network becomes “Tide Locked”, causing flooding to streets and properties. This was the case during the 2021 Flash Floods, even in Maida Vale, where a significant Flood Alleviation Infrastructure Scheme costing £17.5m was quickly overwhelmed by tide-locked sewers.
We would suggest the challenge for Communal Gardens, which represent over 200 Acres of London’s Green/permeable area, is how to reduce rain-water run off contributing to flash flooding in resident’s homes, whilst also making a positive impact to the wider neighbourhood.
Sustainable Drainage Systems (SuDS) or Sustainable Urban Drainage Systems are a collection of Water Management practices that align modern drainage systems with natural water processes, and lessen the burden imposed on sewer networks. Ground infiltration of Water is a clear part of SuDS, and as such, so is Soil Injection.
The Sponge City Concept is an urban planning and design approach that aims to make cities more resilient to climate change impacts, particularly extreme rainfall and urban heat, by mimicking natural hydrological processes. The idea is to transform urban areas into “sponges” that can absorb, store, filter, and slowly release rainwater, rather than relying solely on traditional “grey” infrastructure like pipes and drains to channel water away quickly.
This concept originated in China, notably championed by Professor Kongjian Yu, in response to widespread urban flooding, water scarcity, and the urban heat island effect exacerbated by rapid urbanization and extensive impermeable surfaces.
London authorities (like the Greater London Authority – GLA) and environmental organizations widely acknowledge that transforming London into a “sponge city” is crucial for tackling climate change impacts, particularly flash flooding and water pollution.
However London has a long way to go, a study by Development Firm Arup comparing London to 7 cities (Auckland, Mumbai, Nairobi, New York, Shanghai, Singapore and Sydney) found London was the second-least “spongey”, with only Sydney performing worse.
Soil Injections vs French Drains
Several Garden Squares in London have turned to French Drains after being plagued by years of Flash Flooding.
French Drains work by creating a series of Trenches in the garden within which permeable pipes are run, following a shallow gradient towards an underground tank, typically at the lowest point of the Garden. The Trenches are lined with a strong permeable weed-proof fabric, and backfilled with large free-draining stones, before being filled with a thin layer of soil/grass, level with the surrounding garden’s surface.
The water collected in the tank then needs to be discharged, into:
- the Sewer Network – though Thames Water thought to be increasingly resistant to this
- through an irrigation system – after a complex filtering process to remove silt which would otherwise obstruct irrigation heads
- a soakaway field or raingarden (the only SuDS methods) – this is currently impractical in London given its built up nature, the high clay content of most soil, and the solid impermeable bedrock underlying it
The typical concerns with regards to French Drains are regarding the cost, upfront coordination effort (Planning Applications, Discharge Agreements with Local Authorities, etc), ongoing maintenance requirements (to remove silt) and physical disruption to the Garden during works (trench digging) and visual disruption thereafter (discolour of lawn above drainage channels).
From an environmental standpoint, French Drains are at odds with the sponge city initiative. They rarely improve the net moisture absorption / retention ability of the Garden, and in the worst instances, contribute to the overwhelming of Sewer Networks during flash flooding events.
In summary, whilst representing a highly effective way of removing surface water, and storing a finite amount of said water, French drains are not without their drawbacks.
Soil Injection
- Can be LESS than £10/sqm
- No visible disruption to Garden during / after works (no impact on use / visual enjoyment)
- No Maintenance Required (scarifying encouraged)
- Root Protection Area beneficial / compliant
- No obstruction to future works / services in Garden
- Boosts absorption-contribution of the Garden to the Neighbourhood
- SuDS supporting in Urban Environments
Vs.
French Drains
- Usually More than £50/sqm
- Physical disruption during works, discolouring of grass around drainage channels after
- Annual maintenance / silt removal required
- Root Protection Area infringing / planning permission required
- Difficult to run services / conduct works in vicinity of drains.
- Worsens soil absorption (drying effect) and can overload sewer network
- Rarely SuDS aligned in Urban Environments
Soil Injection for Tree Recovery
The Origins of Soil Injection for Tree Health
As explained in this video interview with Tony Kirkham, the then Head of Arboretum at the Royal Botanical Gardens, Kew: the origins of “Decompacting” tree roots can be traced back to the then-nearly 200-year old Turner’s Oak, and the Great Storm of 1987.
Having suffered years of decline, the whole tree, root plate and all was lifted out of the ground during the storm. Pained to lose it, the Arborists at Kew decided to place the tree back in the ground, propping it with metal struts, with the intention to deal with it last out of all the damages trees that may require felling following the storm.
Much to the team’s amazement, on finally returning to the tree 3 years later it was the picture of health, much improved from its pre-storm condition. The Turner’s Oak has since put on 30% of its growth in the 30 years following the storm.
As shown at 3:25 in the video, the Kew Gardens Team now run their own fleet of soil injection and aeration tools to maintain the health of their c.14,000 tools.
Try Before You Fell: The Financial and Environmental Cases for Soil Injection Before Pollarding / Felling
Unfortunately the majority of Tree Surgeons and Tree Protection Officers will quickly advocate pollarding or felling a poorly tree, with little consideration for Soil Injection, despite clear academic support for the process, including in the few studies of soil injecting urban trees.
The challenge with pollarding or felling mature, large Trees when there is no imminent risk of structural collapse, is it makes little environmental or economic sense:
- Environmental Considerations:
– Pollarding a large mature tree removes its tree canopy, compromising its ability to combat the Urban Heat Island Effect
– It also severely reduces the Tree’s ability to remove pollutants. For example a Plane Tree with a thick canopy can absorb:
— Absorb Particulate Matter (PM10, PM2.5) through its large
— Ozone taken up through stomata; also indirectly reduced by intercepting ozone precursors like NOx and VOCs
— Sulphur Dioxide absorbed through the leaves
— CO2 removed through photosynthesis - Financial
– Pollarding a mature Plane Tree can cost upwards of £3,000 – though if the tree continues to decline these works will not be entirely offset against the cost of felling
– Felling a mature Plane Tree can cost upwards of £15,000, before considering the cost of a mature full-standard replacement tree that might be mandated by the Local Authority
Against this backdrop, spending a few thousand pounds on soil-injecting the entire root protection area of a struggling, though structurally sound large tree, makes both environmental and financial sense.
Enriched Biochar: The Added Ingredient Super powering Soil Injection for Trees
Since the learnings from the Turner’s Oak at Kew Gardens, Arboriculturists and Soil Injection Tool Manufactures have been making great advances in combining the typical root decompaction/aeration process, with injected matter optimised for Tree Health. Enriched Biochar is the preferred material now used across the Soil Inject Industry for tree health.
Biochar is a carbonaceous solid commonly used for soil improvement, that’s produced by heating organic material (like wood, crop residues, or manure) in a low-oxygen environment—a process called pyrolysis, that minimises the release of CO2 emissions.
The process produces solid carbon (biochar) that’s a very stable dry material ideal for soil injection, given its propensity to increase moisture retention, enhance soil aeration and support microbial life.
Enriched Biochar, takes Biochar, and mixes it with Mycorrhizal Fungi, Trichoderma, Wormcasts and Seaweed to boost the health of the surrounding soil, and maximise the chance of a positive improvement in the health of surrounding trees.
The Environmental Case for Biochar - Brining-In Additives Can Benefit Both Your Garden and the Wider Environment
Unfortunately the domination of non-interference mantras, such as “no dig”, in most horticultural publications over the last decade has made many landlords reluctant to bring in external materials/fertilisers to their Garden.
This is particularly to the detriment of Urban Green Spaces, where the “natural ecosystem” argument tends to overlook the constant detrimental impact of human/urban-generated challenges, including Pollution, the removal of Green Waste, the Urban Heat Island Effect, the removal and repurposing of nearby streams (for sewers) and the leaching away of water by the neighbouring ground under non-permeable surfacing.
As such we consider it important to dedicate a subsection for this message – there is a clear environmental case for the use of Biochar:
- Long-term carbon storage (carbon sequestration):
– Plants absorb CO2 from the atmosphere as they grow
– Turning this Plant Matter into Biochar locks up that carbon in a solid form that can last hundreds of thousands of years in the soil
– Without bichoar, that biomass would eventually rot or burn, releasing CO2 and/or methane back into the Atmosphere - Improved soil health = and less need for synthetic fertilisers:
– Biochar improves nutrient and water retention.
– This can reduce the need for nitrogen fertilizers, whose production and overuse generate greenhouse gases like N₂O, which is ~300× more potent than CO₂. - Less Methane and Nitrous Oxide from Soils
– Studies show biochar can suppress emissions of these potent greenhouse gases from wet or compacted soils. - Waste management & Circular Economy
– Biochar production uses agricultural waste that might otherwise be burned, dumped, or left to rot.
– This turns waste into a beneficial product reducing environmental harm
All of which is before considering the well documented improvement Biochar can make to a tree’s health, in turn improving its environmental contribution in terms of absorbing pollutants and combating the Urban Heat Island Effect.
What Differentiates GardenSqua.re
Research-Led Proposals
Ahead of putting forward a quote we will typically conduct thorough research in-house, including assessing nearby borehole records and maps covering topology, geology and flood-risks, allowing us to formulate a better estimate of the areas that will most-benefit from soil injection, and potential benefit it might bring.
Where third party consultants are required, be they civil engineers (to assess flooding risk), or qualified Arboriculturists (to assess tree health / structural integrity), we are happy to make introductions to suitable consultants.
Dig-Safe Contractors
Our team is well-versed in running Services, including via use of blind methods such as impact moling.
As part of our quoting process we pull the technical drawings from utility providers covering Power, Gas and Data Networks, ensuring our contractors are safe to proceed with works as planned without risk of injury or cease-works-order resulting from a cable/pipe strike.
Ahead of instructed works, where necessary we conduct further research to identify further utilities (e.g. Water and Sewer Mains) and underground obstacles (e.g. Open Trench Shelters from WW2) that might be present in the Garden at the depths we are operating at.
This combined with our broader understanding of the History of Garden Squares to London, and the differences to expect between Squares/Neighbourhoods, allows us to operate with a higher degree of confidence than our peers.
Tools to Fit the Location
As specialist contractors focussing near-exclusively on Garden Squares, we understand the challenges of working in Garden Squares (particularly with noise generation and access), investing in our own plant machinery selected specifically to work well on such sites, being easy to manoeuvre on site and able fit through narrow gates where that is the only access.
We are well versed in tree protection and ground protection requirements from the both Royal Borough of Kensington and Chelsea, and Westminster. Our teams know how to work in these areas, minimising disruption and ensuring no damage is done.
Local, Responsive and Price Competitive
Our company is based in central London, with the majority of our staff living in Central-West London. We can typically respond to emergencies (including standing water) inside of 24 hours for existing clients.
The aim for GardenSqua.re is to be the company of choice for Garden Squares for any of their non-horticultural needs. We price efficiently as we don’t consider each job as one of many potential jobs we may undertake with each client.
We are proud to maintain excellent relationships with all of our existing clients, and are happy to provide references for any prospective clients.